Determining the optical characteristics of optical components and networks is a critical element to the successful design and operation of optical communications networks. Important characteristics of an optical component or network are the amplitude, phase, and group delay response measured in either transmission or reflection. Polarization dependent characteristics of an optical component or network include polarization dependent loss (PDL) and differential group delay (DGD). The characteristic of group delay is of particular interest because it is relatively difficult to measure using conventional techniques. Group delay is defined as the negative of the derivative (rate of change) of a component or network's phase response (i.e., the phase term φ(ω) of the component transfer function H(ω)=a(ω)exp(jφ(ω)), where a(ω) denotes the amplitude response). That is, group delay is a measure of phase linearity and is defined by the equation:
      τ    g    =            -              ∂        φ                    ∂      ω      
where ω is angular optical frequency (in radians/second).
Group delay is traditionally measured using a known modulation phase-shift method. The modulation phase-shift method is a relative measurement technique that is essentially unaffected by environmental and physical instabilities such as changes in temperature and vibrations. A drawback to the modulation phase-shift method is that its dynamic range is limited because it relies on direct detection.
Group delay has also been measured using various interferometry-based methods. Interferometry-based methods provide better dynamic range than the modulation phase-shift method. However, known interferometry-based methods are extremely sensitive to environmental and physical instabilities such as changes in temperature and vibrations.